The fuel cell technology is attracting attention as a solution to the problem of energy resources, as well as to the issue of global warming due to CO2 emission.
The fuel cell is adapted for electrochemical oxidation of a fuel, such as hydrogen or methanol or any hydrocarbon else in the cell, to effect a direct conversion of chemical energy of the fuel to electrical energy to be taken out.
The fuel cell is thus free from emissions of combustion products of fuel, such as NOX and SOX, and attracts attention as a clean energy source for internal combustion engines such as for automobiles, or for thermal power plants.
There are some types of fuel cells, with a proton-exchange membrane fuel cell (referred herein sometimes to “PEFC”) inclusive, which is now most watched, and developed.
The PEFC has various advantages, such that it is (1) adapted for an operation to be facile in start and stop at low temperatures, (2) allowed to be high in theoretical voltage as well as in theoretical efficiency of conversion, (3) implemented with a liquid-free electrolyte allowing a flexible design of cell structure, such as a vertical type, and (4) configured for an interface between ion exchange membrane and electrode to have a secured three-phase interface as a reaction field to take out an enhanced amount of current, achieving a high density power output.
Among others of the PEFC, the direct type fuel cell, to which a liquid methanol or any hydrocarbon fuel else is directly supplied for power generation, can be predominantly reduced in size in comparison with the hydrogen type that employs a high-pressure hydrogen storage container to be large in scale, and is different from the gas reforming type in that the former does not need any apparatus associated with a fuel reformer, allowing for a simplified configuration of entire system, as well as for facilitated startup and maintenance services, thus having best applications to a compact mobile power supply for drives or power supply for portable devices.
The PEFC thus most-watched yet has many unsolved problems. In particular, techniques of polyelelectrolyte membrane constitute a top challenge.
An electrolyte membrane that has a now widest application is made of a perfluorosulfonic acid polymer, which is typified by the Nafion® film commercially available from Du Pont Co., U.S.A., and has a history, where it has been developed as a membrane having a tolerance to active oxygen that the fuel cell generates at the air electrode (anode as positive-pole). Long endurance tests have not yet revealed a sufficient tolerance.
The principle of operation of a fuel cell includes two electrochemical processes, being an H2 oxidation at the fuel electrode (cathode as negative-pole), and a four-electron reduction of molecular oxygen (O2) shown by formula (A1) below, which produces water.O2+4H++4e−→2H2O  (A1)
Actually, concurrent side reactions occur. Typically, a two-electron reduction of O2 takes place at the air electrode, producing hydrogen peroxide (H2O2), as shown by formula (A2) below.O2+2H+2e−→H2O2  (A2)
Hydrogen peroxide is stable, and has a long life, though weak in oxidizability.
Hydrogen peroxide decomposes, following reaction formulas (A3) and (A4) shown below. When decomposing, it generates radicals, such as hydroxy radical (.OH) and hydroperoxy radical (.OOH). Such radicals, in particular hydroxy radical, are strong in oxidizability, so that even perfluorosulfonated polymer used as an electrolyte membrane may be decomposed in a long use.H2O2→2.OH  (A3)H2O2→.H+.OOH  (A4)
Low-valence ions of transition metal such as Fe2+, Ti3+, or Cu+, if present in the fuel cell, cause a Haber-Weiss reaction, where hydrogen peroxide is one-electron reduced by such a metal ion, generating hydroxy radical.
Hydroxy radical, most reactive among free radicals, has a very strong oxidizability, as is known. If the metal ion is an iron ion, the Haber-Weiss reaction is known as a Fenton reaction shown by formula (A5) below.Fe2++H2O2→Fe3++OH−+.OH  (A5)
Such being the case, metal ions, if mixed in an electrolyte membrane, cause a Haber-Weiss reaction, whereby hydrogen peroxide in the electrolyte membrane is changed into hydroxy radical, whereby the electrolyte membrane may be deteriorated (Kyoto University Graduate School of Engineering as entrustee from the New Energy and Industrial Technology Development Organization, “2001 yearly results report, researches and developments of proton-exchange membrane fuel cell, researches on deterioration factors of proton-exchange membrane fuel cell, find research (1) on deterioration factors, deterioration factor of electrode catalyst/electrolyte interfaces”, March 2002, p. 13, 24, 27).
With that, to prevent an electrolyte membrane from being oxidized by hydroxy radical, there has been a method proposed in Japanese Patent Application Laying-Open Publication No. 2000-223135, for example, in which a compound with phenolic hydroxyl is mixed in the electrolyte membrane, so that peroxide radicals are trapped to be inactive.
Another method is proposed in Japanese Patent Application Laying-Open Publication No. 2004-134269, in which an electrolyte membrane has a phenol compound, amine compound, sulfur compound, phosphorus compound, or the like mixed therein as anantioxidant to vanish generated radicals.
Another method proposed in Japanese Patent Application Laying-Open Publication No. 2003-109623 has an electrolyte membrane disposed adjacent to a catalyst layer containing molecules having a smaller bond energy than carbon-fluorine bonding, the molecules reacting with priority to hydroxy radicals, thereby protecting the electrolyte membrane.